Doubts in IPv6 setup

I am using Pihole for some month, but still I have some doubts in my configuration.
I would like to ask some details.

First one simple question:
I installed pihole with setting IPv6 off.
When I later open setupVars.conf and save an IPv6 address, do I have already IPv6 enabled after a reboot,
or do I have to pass "pihole -r" again to enable v6 ?

Next the big question. Some internet sides are loading slow.
For example this one,

My router is Speedport 724v, and this type is a bit limited in configurations.
Anyway, IPv6 ULA is available and enabled.
The router itself is DHCP.

Now, the router shows me all Raspberry IP addresses as v4 and v6 GUA and ULA.
In the past since some weeks, I saved this shown IPv6 ULA address into the setupVars.conf file.
So that they are corresponding between Router and Pihole.
So far...
Now I connect the Pi sending ifconfig.
It returns the correct given IPv4
but 3 different addresses for IPv6.
fd21 ...(Global)
2003 ...(Global)
fe80 ...(Link)

None of the 3 addresses corresponds with the IP showing up in my routers client info.
Now I dont know, how to setup pinhole correct.
Which address should I best save with the setupVars.conf file.

I did serveral tests using the address showing in the Routers client info,
as well as with the address given via ifconfig, all without noteble changes.
After all, I am remaining with the doubt of an correct setup.

Any suggestion for this case?

Many thanks!

You should add IPv6 by choosing it via pihole -r. We do not recommend manually changing variables inside of setupVars.conf.
As for the slow loading websites, see here:

Thanks for reply!
yes I know this thread but I am confused in electing the right IPv6 address as they
are not corresponding between router and pi.
In the meantime I have disabled IPv6 in pihole and client, cause I'm unsure about that setup.